I am looking to do time-lapses during sunset and sunrise. I have a EOS 600d/T3i. My current intervalometer is pretty cheap and will not work for this. I believe I need a "Bulb Ramping" intervalometer. Does anyone know of a good one and where I might be able to buy it? Thank you!

I have read a little bit on the "Little Bramper", but it says it is "unknown" as far as working for the T3i.

Device is also weather sealed and it's got a large backlit LCD. Promote Remote controls the camera via shutter release cable + USB cable and in case of Bulb Ramping, it is connected to the camera via shutter-cable + PC Sync cable.
P.R. has a complete control over your camera, its auto-focusing mechanism, mirror-locking, etc.

I own pretty much ALL remotes out there and have tested them extensively over the past 2 years.
Promote Remote is a super-robust, professional workhorse for a serious photographer and in terms of features and performance, it is in a league of its own. Promote System's tech support is top notch and firmware updates keep adding the new and exciting functionality to the already impressive list of features.
In short, there's no other device on the market that can even remotely compare to Promote Remote.

Magic Lantern for Canon T3i has support for Bulb Ramping and Intervalometer. Personally I do use Magic Lantern on my 60D and had no problems with it but I have not tried those features yet.
Magic Lantern is a free software that sits on top of your Canon firmware. You can get more information about it on their website.

It looks like the reason Nikon cameras can't do bulb ramping is because their bulb mode only works in discrete steps, and doesn't allow precise control. While the Little Bramper site says that newer Canon Rebel models aren't tested, I don't see any reason to believe that they wouldn't work. There's no sign that Canon has changed how they do things (and I think there would be outcry if they had).

The Arduino-based TriggerTrap doesn't have this functionality, although they are apparently looking at adding it in an update. I mention it, though, because they have an extensive list of supported models and the Rebel T3i is on the list with all the rest with no special note.

So, anyway, I think you will be safe with the Little Bramper. There's apparently a 30-day no-questions return policy, so if it doesn't work you'll only be out the (note: rather high) cost of shipping. I think it's a decent gamble.

Photix has a new intervalometer with features that seem equivalent to the Promote for about 1/3 of the cost. I just picked one up and I'm starting to experiment with it, and it looks like it'll get the job done for me without spending as much; it's also got cables so it'll work with both the 7d and t3i with the same unit (unlike Canon's units), which is very convenient for me...

with my early looks at the Aion, it seems to have good build quality and other than an instruction manual that is pretty useless, the hardware seems to be a good value.

The best solution to your problem can be solved using the Magic Lantern firmware addon for the many different Canon versions including Canon 500/550 and 600D that was mentioned previously and a free lightroom extension called LRTimelapse